The present invention relates to a circuit suitable for ICs used in VIRs and audio equipment, and more particularly to a switching circuit for AC bias signal.
The conventional record/playback circuit in a tape recorder is known as shown in FIG. 1. In this circuit, including a couple of switching circuits, the playback mode will be first described. One switching circuit SW1 is ON, while the other switching circuit SW2 if OFF. A magnetic head 1 picks up a signal recorded on a magnetic tape 3 to produce a playback signal. The playback signal is input to a playback preamplifier 5, via a coupling capacitor C1. The preamplifier 5 amplifies the playback signal to a level strong enough to drive a loudspeaker. Since the positive and negative peak voltages of the playback signal are several mV, a large withstand voltage is not required in the design for a switching circuit SW2. In the record mode, the switching circuit SW1 is OFF, while the switching circuit SW2 is ON. A recording signal is input from a microphone into the record/playback circuit passes through a recording amplifier 11 and a coupling capacitor C1, to the magnetic head 1. The recording signal is superposed with an AC bias signal output through a capacitor C3 and a resistor R1, and the superposed signal is applied to the magnetic head 1. The bias signal has positive voltage of 50 V and negative peak voltages of -50 V. Accordingly, a 50 V peak voltage and a -50 V peak voltage are alternately applied to the switching circuit SW1 when it is OFF. For this reason, the switching circuit SW1 must be designed so as to withstand the peak to peak voltage of 100 V or more, otherwise, the recording signal is distorted.
The switching circuit SW1 arranged using a discrete component is shown in FIG. 2. As shown, the collector of a transistor 15 is connected to an input terminal Tin. The emitter of the transistor 15 is connected to the emitter of a transistor 17. The collector of the transistor 17 is connected to a reference voltage terminal Tref. The ground potential is normally applied to the reference voltage terminal Tref. For controlling the switching operation, a control signal S is applied to the bases of the transistors 15 and 17. When the switching circuit of FIG. 2 is fabricated into an integrated circuit, parasitic diodes D1 and D2 are inevitable formed between the collectors of the transistors 15 and 17, and the substrate, as shown in FIG. 3. For increasing the withstand voltage of the switching circuit SW1 to a larger value, it is ideal to electrically open the substrate. However, the semiconductor substrate is not generally used in an open state. When a signal applied to the input terminal Tin is negative value, the voltage of the signal is clamped at a value of the forward voltage of the parasitic diode D1 lower than the voltage of the semiconductor substrate. For the above reasons, it is very difficult to fabricate the switching circuit of FIG. 2 into an integrated circuit. The general integrated switching circuit is shown in FIG. 4. As shown, the collector of a transistor 19 is connected to the input terminal Tin, and the emitter thereof is connected to the reference voltage terminal Tref. The collector of the transistor 21 is coupled with the base of the transistor 19. The emitter of the transistor 21 is connected to the semiconductor substrate. For controlling the switching operation, a control signal S is applied to the base of the transistor 21. In the arrangement shown in FIG. 4, the voltage Vsub of the semiconductor substrate must be lower than the voltage V.sub.Tref at the reference voltage terminal Tref. If the base-emitter voltage of the transistor 19 is V.sub.BE(19), and the collector-emitter saturation voltage of the transistor 21 is V.sub.CE(sat)(21), these voltages must satisfy the following relations. EQU V.sub.Tref &gt;V.sub.sub +V.sub.BE(19) +V.sub.CE(sat)(21) ( 1)
Accordingly, for setting the reference voltage terminal Tref of the switching circuit shown in FIG. 4 at ground level, the voltage of the semiconductor substrate must be set negative value. This necessitates two power sources. One of them generates positive voltage and the other of them generates negative voltage. When an offset voltage is applied to the reference voltage terminal Tref for setting the substrate voltage to ground level, a power source for offset voltage is additionally needed. Further, when the switching circuit, where the reference voltage terminal Tref is set at the offset voltage, is used as the switching circuit SW1 in FIG. 1, DC voltage is always applied to the magnetic head 1. So the magnetic head 1 is subject to electrolytic corrosion.